In the presence of an appropriate electromagnetic field, solid particles in rheological fluids move into alignment. When this alignment occurs, the ability of the fluid to flow, or be sheared, is substantially decreased. In the presence of an energy field rheological fluids respond by forming fibrous structures parallel to the applied field. The formation of these fibrous structures triggers a significant increase in the viscosity of the fluid, by factors as high as 10.sup.5. This phenomenon has been observed to occur in the presence of both magnetic fields and electrical fields. Hence the terminology "electrorheological fluid" (E.R. fluid) and "magnetorheological fluid" (M.R. fluid).
Electrorheological fluids and magnetorheological fluids comprise a carrier medium, such as a dielectric medium, including mineral oil or silicone oil, and solid particles. Magnetorheological fluids require the use of solid particles that are magnetizable, and electrorheological fluids make use of solid particles responsive to an electric field.
The reversible magnetic-field-induced solidification of slurries of micron-sized magnetizable particles was apparently first reported by Rabinow of the National Bureau of Standards in the late 1940s. Rabinow utilized the controllable rheological properties exhibited by these magnetorheological fluids to construct a series of novel electromechanical clutches.
Prior to that time, Winslow discovered that a similar effect was produced by the application of high electric fields to electrorheological fluids.
These early investigations led to three main discoveries regarding rheological fluids: 1) In the presence of an energy field the particulate matter within the fluid fibrillates and highly elongated condensed structures of particles form parallel to the field; 2) A stress often exponentially related to the field is necessary in order to shear the fluid; consequently, at low shear stresses, the system resembles a solid; 3) At stresses greater than yield stress, the fluid flows like a viscous fluid.
The basis for the magnetorheological effect can be explained by the interparticle force induced by an applied magnetic field. Most M.R. materials are comprised of magnetizable powders--iron, steel, nickel, cobalt, ferrites and garnets--having particle sizes large enough (say 0.1 to 100 micrometers) to incorporate a multiplicity of magnetic domains. As a result, the particles possess little or no permanent magnetic moment but are readily magnetized by an applied magnetic field (H.sub.o). The level of magnetic induction B induced in the bulk material is characterized by its relative permeability .mu..sub.p such that B=.mu..sub.o .mu..sub.p H.sub.o, where .mu..sub.o =1.2.times.10.sup.-6 H/m is the permeability of free space. The relative permeability is itself a function of the applied field in non-linear materials such as those commonly used in M.R. applications. The initial or low-field permeability of carbonyl iron is commonly reported to be about 100.
When an external magnetic field is applied to an initially random arrangement of magnetizable particles, a magnetic moment (roughly) parallel to the applied field is induced in each particle. The force between two particles whose moments are aligned head-to-tail is attractive, promoting the formation of chains or more complicated networks of nearly contacting particles aligned along the direction of the field. The network of particles so formed is essentially a solid. It can support a shear stress without flowing on laboratory time scales.
The strength of this solid can be characterized by the yield shear stress .tau..sub.y, the stress at which the network is disrupted and the particles flow. The yield stress is an important figure-of-merit for device design because, e.g., a high yield stress enables the solidified fluid to sustain larger mechanical forces before flowing. The yield stress is a direct consequence of the interparticle forces and so its order of magnitude can be estimated on the basis of the magnetic forces between the magnetized particles.
One estimates that the yield stress for highly permeable particles is proportional to .mu..sub.o .mu..sub.f H.sub.o.sup.2, where .mu..sub.f is the relative permeability of the suspending medium. The relative permeabilities of the various suspending fluids found in the prior M.R. art are all essentially unity.
The ability of these slurries to reversibly solidify in the presence of an energy field offered scientists a controllable mixture for use in the field of servo-mechanisms and other electromechanical applications. Such applications require field-responsive fluids to have very low shear resistance at zero field, high shear stresses at maximum applied field, very low hysteresis, chemical inertness, temperature stability and fast response time.
In contrast to both M.R. fluids and E.R. fluids, ferrofluids are fluids which typically consist of colloidal magnetic particles, such as magnetite and manganese-zinc ferrites, dispersed in a continuous carrier phase. The average particle size of the magnetic particles dispersed in ferrofluids ranges between 5-10 nm.
Upon the application of a magnetic field, colloidal magnetic fluids retain their liquid properties. Due to the effect of Brownian motion on the polarized particles, ferrofluids do not generally exhibit the ability to form particle fibrils or develop a yield stress. On the contrary, ferrofluids experience a body force on the entire fluid which is proportional to the magnetic field's gradient. This force causes ferrofluids to be attracted to regions of high magnetic field strength.
The similarity between M.R. fluids and ferrofluids has caused some confusion in the literature. Ferrofluids are not continuous media, although they often can be treated as such. Rather, they consist of very small (diameters are typically less than 10 nm) single-domain magnetic particles (often magnetite, Fe.sub.3 O.sub.4) dispersed with the aid of surfactants into various fluid media like water or oils. The particles found in M.R. fluids are orders of magnitude larger in size. Unlike M.R. fluids, ferrofluids do not solidify in an applied field, though they do exhibit field-induced viscosity increases. See, e.g. J. P. McTague, J. CHEM. PHYS. 51, 133 (1969). Ferrofluids do experience body forces in homogeneous magnetic fields, allowing their position to be manipulated, thus enabling the construction of ferrofluid devices like rotary seals and vacuum feedthroughs.
Electrorheological fluids (E.R. fluids) have in application presented a variety of problems. For example, E.R. fluids exhibit low yield strength and temperature sensitivity, which thereby severely limits their use in most applications. Furthermore, the inability of E.R. fluids to withstand water contamination has posed a serious issue in terms of compatible applications. Lastly, a large scale application of E.R. fluid systems requires high voltage power supplies which are both potentially dangerous and expensive. Despite these problems, the need for a controllable mixture has led scientists to use E.R. fluids in shock absorbers, clutches, engine mounts, and active bushings.
Magnetorheological fluids (M.R. fluids) offer a similarly controllable mixture having almost none of the problems associated with E.R. fluids. M.R. fluids generally consist of micron-sized, magnetically polarizable particles dispersed in a carrier medium. The formation of fibrous structures upon the application of a magnetic field is essential to the operation of a M.R. fluid. In the presence of a shear force, the equilibrium that is established between the formation and breaking of particle fibrils determining the yield strength for the fluid.
An important technical measure of magnetorheological fluid is its yield stress. Yield stress is defined as the applied stress required for the fibrils to flow or `yield`; it generally increases with an increase in the energy field strength. Accordingly, yield stress defines the onset of flow.
Notably, the yield stress values generated by M.R. fluids are significantly greater than those measured for their E.R. fluid counterparts. In fact, yield stress values in excess of 80 kPa are easily obtainable for M.R. fluids in the presence of a magnetic field. As a comparison, while yield stress values for M.R. fluids are typically 100 kPa, yield stresses of E.R. fluids are 10 kPa at best.
An additional advantage of M.R. fluids over E.R. fluids exists in the ability of M.R. fluids to operate over a broad temperature range. M.R. fluids are reported to function effectively throughout the temperature range of -40 to 150.degree. Celsius. Over this 190.degree. range, only a small variation in the yield strength of the M.R. fluid can be observed.
Lastly, M.R. fluids can utilize low voltage, current-driven power supplies, which currently exist for large volume applications at a relatively low cost.
Thus, the main advantages associated with M.R. fluids over E.R. fluids include the ability to obtain very high yield stress values, the compatibility of M.R. fluids to low voltage, current-driven power supplies available in large volumes and a reduced sensitivity to the presence of low level contaminants. Additionally, M.R. fluids respond to the application of a magnetic field on the order of milliseconds.
Devices using rheological fluids may be simpler and more reliable than electromechanical devices due to the reduction of moving parts necessary. These fluids offer an efficient means to interface mechanical components with an electrical control system. Thus, rheological fluids have been used for controllable dampers, mounts, clutches, and brakes through the rapid response of these fluids to changes in their applied field.
A publication entitled "Magnetorheological Effect For Non-Magnetic Particles Suspended In A Magnetic Liquid" by Kashevskii et al., (1989) discusses the rheology of "magnetic hold" suspensions. The publication discusses the combination of nonmagnetic, non-colloidal particles with a magnetic liquid and the resultant magneto-rheological effect. There is no suggestion of a magnetic fluid composition comprising magnetizable particles in a magnetizable liquid.